Filtering techniques are very varied. However, when frequency and/or power are high, rectangular or cylindrical waveguide filtering remains the only means for obtaining good performance with respect to loss and to selectivity.
When the relative pass band is less than about 2%, circular cavity filters have exceptional characteristics since they operate with very high Q factors.
For wider pass bands, three types of filter are currently in use:
evanescent mode or comb type filters, this low Q factor technique provides wide band filters with a Q factor .ltoreq.1000;
suspended coaxial filters, these have the same performance as the above filters with the advantage of precision due to the principle of thin layer etching; and
waveguide filters.
A TE101 or "E-plane" filter is currently useable for relative passbands up to 5% or 6%. Beyond that, end coupling susceptances become very low and in practice, physical embodiments become very difficult if not impossible.
An article entitled "Broadband Millimeter Wave E-plane Bandpass Filters in IEEE MTT-S Digest (1984, pp 236-237) describes a method of designing an E-plane bandpass filter having pass bands which are greater than 10% and in which the structure used is a so-called "ladder" structure.